diff --git a/core/modules/block/block.admin.inc b/core/modules/block/block.admin.inc
deleted file mode 100644
index 86c59a8..0000000
--- a/core/modules/block/block.admin.inc
+++ /dev/null
@@ -1,27 +0,0 @@
-<?php
-
-/**
- * @file
- * Admin page callbacks for the block module.
- */
-
-use Drupal\block\Entity\Block;
-use Drupal\Core\Template\Attribute;
-
-/**
- * Page callback: Attaches CSS for the block region demo.
- *
- * @see block_menu()
- *
- * @deprecated Use \Drupal\block\Controller\BlockController::demo()
- */
-function block_admin_demo($theme = NULL) {
-  return array(
-    '#attached' => array(
-      'library' => array(
-        array('block', 'drupal.block.admin'),
-      ),
-    ),
-  );
-}
-
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index f1112d6..d0134b9 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -135,7 +135,6 @@ function block_menu() {
       'route_name' => "block.admin_display_$key",
     );
     $items["admin/structure/block/demo/$key"] = array(
-      'title' => check_plain($theme->info['name']),
       'route_name' => 'block.admin_demo',
       'type' => MENU_CALLBACK,
       'theme callback' => '_block_custom_theme',
@@ -150,7 +149,6 @@ function block_menu() {
  *
  * Path:
  * - admin/structure/block/list/% (for each theme)
- * - admin/structure/block/demo/% (for each theme)
  *
  * @param $theme
  *   Either the name of a theme or a full theme object.
@@ -193,9 +191,10 @@ function block_page_build(&$page) {
 
   // Fetch a list of regions for the current theme.
   $all_regions = system_region_list($theme);
-
   $item = menu_get_item();
-  if ($item['path'] != 'admin/structure/block/demo/' . $theme) {
+  // Make sure the blocks are not getting rendered for demo page
+  // or theme specific site building page.
+  if ($item['path'] != 'admin/structure/block/demo/%' || !isset($item['map'][4]) || $item['map'][4] != $theme) {
     // Load all region content assigned via blocks.
     foreach (array_keys($all_regions) as $region) {
       // Assign blocks to region.
diff --git a/core/modules/block/lib/Drupal/block/Controller/BlockController.php b/core/modules/block/lib/Drupal/block/Controller/BlockController.php
index c5c3fc3..7d566b4 100644
--- a/core/modules/block/lib/Drupal/block/Controller/BlockController.php
+++ b/core/modules/block/lib/Drupal/block/Controller/BlockController.php
@@ -7,17 +7,33 @@
 
 namespace Drupal\block\Controller;
 
+use Drupal\Core\Controller\ControllerBase;
+use Symfony\Component\HttpFoundation\Request;
+
 /**
- * Controller routines for block routes.
+ * Controller routines for admin block routes.
  */
-class BlockController {
+class BlockController extends ControllerBase {
 
   /**
-   * @todo Remove block_admin_demo().
+   * Returns a block theme demo page.
+   *
+   * @param string $theme
+   *   The name of the theme.
+   *
+   * @return array
+   *   A render array containing the CSS and title for the block region demo.
    */
   public function demo($theme) {
-    module_load_include('admin.inc', 'block');
-    return block_admin_demo($theme);
+    $themes = list_themes();
+    return array(
+      '#title' => String::checkPlain($themes[$theme]->info['name']),
+      '#attached' => array(
+        'library' => array(
+          array('block', 'drupal.block.admin'),
+        ),
+      ),
+    );
   }
 
 }
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php
index d996b1e..48c46c3 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php
@@ -68,6 +68,17 @@ function setUp() {
   }
 
   /**
+   * Test block demo page exists and functions correctly.
+   */
+  public function testBlockDemoUiPage() {
+    $this->drupalPlaceBlock('system_help_block', array('region' => 'help'));
+    $this->drupalGet('admin/structure/block');
+    $this->clickLink(t('Demonstrate block regions (@theme)', array('@theme' => 'Stark')));
+    $elements = $this->xpath('//div[contains(@class, "region-highlighted")]/div[contains(@class, "block-region") and contains(text(), :title)]', array(':title' => 'Highlighted'));
+    $this->assertTrue(!empty($elements), 'Block demo regions are shown.');
+  }
+
+  /**
    * Test block admin page exists and functions correctly.
    */
   function testBlockAdminUiPage() {